Removal of epileptogenic brain areas in which seizures originate offers patients with refractory epilepsy the chance of being seizure free. However, this resection must be balanced against the preservation of eloquent cortical areas to reduce postoperative morbidity. Many screening technologies based on the occurrence of both inter-ictal and ictal abnormal activities have been used during the preoperative evaluation phase for epilepsy surgeries, including electroencephalograms (EEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and single-photon emission computed tomography (SPECT). Together, they provide the general location but not the exact boundaries of seizure-inducing brain areas. In addition, the usefulness of information provided by these non-invasive techniques degrades during the course of surgery because of brain shifting and deformation that is invariably induced by the loss of cerebrospinal fluid.
Electrocorticography (ECoG) has been demonstrated to be a valuable intraoperative tool, not only to precisely delineate such boundaries, but also to identify eloquent cortical areas. However, the technique requires prolonged recordings with electrodes implanted long-term, thereby elevating the risks of hemorrhage, infection and cerebral edema. Intraoperative MRI (iMRI) and fMRI (ifMRI) have recently started to play crucial roles in epilepsy surgery, as they enable the maximum extent of resection despite the lesion's proximity to eloquent brain cortex and fiber tracts which, in turn, leads to favorable seizure-reduction outcomes and acceptable neurological deficit rates. Unfortunately, the use of iMRI or ifMRI demands an extremely high standard of infrastructure and maintenance. As a result, only a limited number of hospitals and research institutes have the financial and technical capabilities to offer these technologies for routine patient care. In addition, the functional mapping of ifMRI relies on the selection of hemodynamic response functions, which could compromise the accuracy of localization.